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| <title>The C Preprocessor: Preprocessor Output</title> |
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| <a name="Preprocessor-Output"></a> |
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| Next: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="n" rel="next">Traditional Mode</a>, Previous: <a href="Other-Directives.html#Other-Directives" accesskey="p" rel="prev">Other Directives</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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| <a name="Preprocessor-Output-1"></a> |
| <h2 class="chapter">9 Preprocessor Output</h2> |
| |
| <p>When the C preprocessor is used with the C, C++, or Objective-C |
| compilers, it is integrated into the compiler and communicates a stream |
| of binary tokens directly to the compiler’s parser. However, it can |
| also be used in the more conventional standalone mode, where it produces |
| textual output. |
| </p> |
| <a name="index-output-format"></a> |
| <p>The output from the C preprocessor looks much like the input, except |
| that all preprocessing directive lines have been replaced with blank |
| lines and all comments with spaces. Long runs of blank lines are |
| discarded. |
| </p> |
| <p>The ISO standard specifies that it is implementation defined whether a |
| preprocessor preserves whitespace between tokens, or replaces it with |
| e.g. a single space. In GNU CPP, whitespace between tokens is collapsed |
| to become a single space, with the exception that the first token on a |
| non-directive line is preceded with sufficient spaces that it appears in |
| the same column in the preprocessed output that it appeared in the |
| original source file. This is so the output is easy to read. |
| See <a href="Differences-from-previous-versions.html#Differences-from-previous-versions">Differences from previous versions</a>. CPP does not insert any |
| whitespace where there was none in the original source, except where |
| necessary to prevent an accidental token paste. |
| </p> |
| <a name="index-linemarkers"></a> |
| <p>Source file name and line number information is conveyed by lines |
| of the form |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"># <var>linenum</var> <var>filename</var> <var>flags</var> |
| </pre></div> |
| |
| <p>These are called <em>linemarkers</em>. They are inserted as needed into |
| the output (but never within a string or character constant). They mean |
| that the following line originated in file <var>filename</var> at line |
| <var>linenum</var>. <var>filename</var> will never contain any non-printing |
| characters; they are replaced with octal escape sequences. |
| </p> |
| <p>After the file name comes zero or more flags, which are ‘<samp>1</samp>’, |
| ‘<samp>2</samp>’, ‘<samp>3</samp>’, or ‘<samp>4</samp>’. If there are multiple flags, spaces |
| separate them. Here is what the flags mean: |
| </p> |
| <dl compact="compact"> |
| <dt>‘<samp>1</samp>’</dt> |
| <dd><p>This indicates the start of a new file. |
| </p></dd> |
| <dt>‘<samp>2</samp>’</dt> |
| <dd><p>This indicates returning to a file (after having included another file). |
| </p></dd> |
| <dt>‘<samp>3</samp>’</dt> |
| <dd><p>This indicates that the following text comes from a system header file, |
| so certain warnings should be suppressed. |
| </p></dd> |
| <dt>‘<samp>4</samp>’</dt> |
| <dd><p>This indicates that the following text should be treated as being |
| wrapped in an implicit <code>extern "C"</code> block. |
| </p></dd> |
| </dl> |
| |
| <p>As an extension, the preprocessor accepts linemarkers in non-assembler |
| input files. They are treated like the corresponding ‘<samp>#line</samp>’ |
| directive, (see <a href="Line-Control.html#Line-Control">Line Control</a>), except that trailing flags are |
| permitted, and are interpreted with the meanings described above. If |
| multiple flags are given, they must be in ascending order. |
| </p> |
| <p>Some directives may be duplicated in the output of the preprocessor. |
| These are ‘<samp>#ident</samp>’ (always), ‘<samp>#pragma</samp>’ (only if the |
| preprocessor does not handle the pragma itself), and ‘<samp>#define</samp>’ and |
| ‘<samp>#undef</samp>’ (with certain debugging options). If this happens, the |
| ‘<samp>#</samp>’ of the directive will always be in the first column, and there |
| will be no space between the ‘<samp>#</samp>’ and the directive name. If macro |
| expansion happens to generate tokens which might be mistaken for a |
| duplicated directive, a space will be inserted between the ‘<samp>#</samp>’ and |
| the directive name. |
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| Next: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="n" rel="next">Traditional Mode</a>, Previous: <a href="Other-Directives.html#Other-Directives" accesskey="p" rel="prev">Other Directives</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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